Ring laser gyros have been developed to provide an alternate form of rotational measurement to the mechanical gyroscope by use of an all optical system. A basic two-mode ring laser gyroscope has two independent counter-rotating light beams oscillating in an optical ring cavity. The frequencies of the light beams depend on the rotation rate of the cavity with respect to inertial frame of reference. In this manner, the rotation rate is proportional to the beat note. Ideally, the ring laser gyro contains no moving parts. In practice, however, the two-mode laser gyro often must be mechanically dithered to keep the counter-rotating traveling waves from locking at low rotation rates.
To improve on this design and avoid the need for a dither, non-planar gyros have been invented. As an alternative to the use of dithering, investigations have shown that the "lock-in" problem can be eliminated by using a ring cavity that contains more than one pair of counter rotating modes. The operation of a basic four-frequency laser gyroscope is described in U.S. Pat. No. 3,741,657 issued June 26, 1973, to K. Andringa. In such four-frequency laser gyroscope systems, beams of four distinct frequencies propagate around a closed propagation path defined by three or more mirrors. Two of these beams circulate around the closed propagation path in a clockwise direction while the other two circulate in the anti-clockwise direction. One of the clockwise beams and one of the anti-clockwise beams are of a first polarization sense, while the other one of the clockwise and the other one of the anti-clockwise beams are of another polarization sense. For example, the first clockwise beam and the first anticlockwise beam may be of right circular polarization while the second clockwise and the second anticlockwise beams may be of a left hand circular polarization. The two right hand circular polarized beams may be of the highest two frequencies while the left hand circular polarized beams may be of the lowest two frequencies.
Rotation of the multi-mode laser gyroscope about its central axis causes the two right hand circular polarized beams to move further apart in frequency than at rest state while the two left hand circular polarized beams become closer together in frequency. Opposite frequency shifts occur for opposite direction of rotation. The difference between frequency shifts in the right hand circular polarized beam and the left hand circular polarized beam is in direct proportion to the rate of rotation of the system. The time integral of this difference is directly proportional to the total amount of rotation about the sensitive axis.
In the system described in the Andringa '657 Patent, a quartz crystal rotator provides the necessary optical activity to cause a split between the average of the frequency of the right and left hand circularly polarized beams. The split accomplished by this crystal provides a phase delay for circular polarized waves that is different for one sense of circular polarization than for the opposite sense and is a reciprocal split. In addition to an element for reciprocal splitting, a planar multi-mode oscillator ring laser gyro may also have a Faraday rotator which provides frequency split between the sets frequencies of clockwise and anticlock wise beams of both left and right polarization. The Faraday rotator is a non-reciprocal device providing different phase delay for waves of the same polarization states propagating in opposite directions.
Among the multi-mode ring laser gyros, a non-planar configuration comprising at least four mirrors and a non-reciprocal rotator is described in Smith, U.S. Pat. No. 4,548,501, issued Oct. 22, 1985. In a non-planar configuration, reciprocal rotation is accomplished by the non-planar geometry of the multi-mode ring laser gyro. The out-of-planeness geometry in a folded rhombus ring laser gyro, provides the necessary reciprocal splitting into left and right circular polarized beams. However, the clockwise and anticlockwise component of each circularly polarized beam are essentially locked, even if the mirror surfaces were perfect. In order to further split the right and left circular beams into their clockwise-anticlockwise frequency components, a nonreciprocal rotator means, such as a Faraday Rotator is used. The left and right circularly polarized sets of beams are widely separated in frequency. In this manner a multi-mode ring laser gyro avoids the problem of mode lock in common to a two-mode ring laser gyro.
However, a phenomenon known as "scatter coupling" still occurs between the clockwise and anti-clockwise members of each set of frequencies of the right and left handed circularly polarized beams. The Faraday rotator splits the clockwise and anticlockwise components of the left and right circularly polarized beams apart in frequency. In a multi-mode setting, typically, there are two lower frequency left circularly polarized clockwise and anticlockwise modes and two higher frequency, right circularly polarized clockwise and anticlockwise modes.
Typically, the counter-rotating modes of left and right circularly polarized beams are separated by about 1 MHz, while about 100 to 1,000 MHz separates left and right polarization. Unlike a planar two-mode ring laser gyro, where "lock-in" characteristics improve with the rate of rotation of the ring laser gyro, in the folded rhombus multi-mode gyro, as the speed of rotation of the ring laser gyro increases, the clockwise and anticlockwise components in a multi-mode ring laser gyro move towards a locking condition. To the extent separation of a clockwise and anticlockwise components is possible, the Faraday rotator provides non-reciprocal splitting in order to establish four separate modes of propagation.
All 4 mirrors and the Faraday rotator scatter some of the incident beams back into the oppositely propagating beams. This is called retroscatter and causes coupling between the beams which generally leads to rotation measurement errors.
As used in this application, the term "retroscatter" refers the phenomenon of light scatter as it occurs between counter propagating beams of the right and left set of beams as these beams are directly incident upon the surface of a mirror or a Faraday rotator. Upon reaching a mirror, a left circularly polarized beam (LCP) is reflected primarily off the surface of the mirror with a shift of helicity from right circularly polarized (RCP) to left circularly polarization in the next leg. Not all the light falling upon a mirror of the ring laser gyro is reflected, some of the light energy and photons are retro-scattered onto the counter-propagating light beam of the same polarity. The anti-clockwise left circular ring laser gyro beam, for example, retro-scatters onto the clockwise ring laser gyro beam and vice versa. In this manner each of the counter-propagating beams have a retro-scatter effect on one another and the phenomenon as a whole is known as "scatter coupling." Although not necessarily as severe a problem as frequency lock in a two-mode laser gyroscope, the scatter coupling effect can hurt the accuracy and precision of a non-planar ring laser gyro. This application seeks to address the problem of scatter coupling.
In a non-planar multi-mode ring laser gyro, there are at least six sites at which scatter coupling may occur between clockwise and anticlockwise components. These sites include each of the four mirrors (four mirrors being the minimum required to achieve a non-planar gyro) and the front and exit surfaces of the non-reciprocal Faraday rotator.